The present invention relates generally to burnable poisons (also called burnable absorbers) for nuclear reactors and, more particularly, to an improved method for coating a nuclear fuel with a burnable poison and to a nuclear fuel coated with a burnable poison by the method of the invention.
It is known that nuclear fuel may have various shapes such as plates, columns, and even fuel pellets disposed in end-to-end abutment within a tube or cladding made of a zirconium alloy or stainless steel. The fuel pellets contain fissionable material, such as uranium dioxide, thorium dioxide, plutonium dioxide, or mixtures thereof. The fuel rods are usually grouped together to form a fuel assembly. The fuel assemblies are arranged together to constitute the core of a nuclear reactor.
It is well known that the process of nuclear fission involves the disintegration of the fissionable nuclear fuel material into two or more fission products of lower mass number. Among other things the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time the fuel assembly with fissionable material must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming and costly, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For first time fueling and subsequent refueling of a thermal reactor, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts to compensate for initial higher reactivity may lead to highly beneficial effects. Such neutron-capturing elements are usually designated as burnable poisons (or burnable absorbers) if they have a high probability (or cross section) for absorbing neutrons while producing no new or additional neutrons or changing into new poisons as a result of neutron absorption. During reactor operation the burnable poisons are progressively reduced in amount so that there is a compensation made with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material as well as a calculated amount of burnable poison. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable poison which undergoes transformation to elements of low neutron cross section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. The burnable poison compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less burnable poison captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fuel and burnable poison in carefully proportioned quantity, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity.
Burnable poisons which may be used include boron, gadolinium, samarium, europium, and the like, which upon the absorption of neutrons result in isotopes of sufficiently low neutron capture cross section so as to be substantially transparent to neutrons.
The incorporation of burnable poisons in fuel assemblies has been recognized in the nuclear field as an effective means of increasing fuel capacity and thereby extending core life. In U.S. Pat. No. 3,427,222 a boron-containing burnable poison layer is fusion bonded to the surface of a nuclear fuel pellet substrate. Such existing techniques for coating nuclear fuel substrates with burnable poisons have problems with the adherence of the coating and with the control of the coating thickness. For boron-containing burnable poisons, existing coating techniques usually require high temperatures well over 600.degree. C.) with the result that cooldown creates substantial strains at the coating-pellet interface due to thermal expansion differences resulting in fracture and hence poor adhesion. Other problems include chemical incompatibility at the coating temperatures resulting in either pellet or coating deterioration.
Conventional coating techniques used in non-nuclear applications have included sputtering. Sputtering is not a fusion bonding process. In U.S. Pat. No. 4,209,375 wear resistant coatings (including boron-containing materials) were sputtered on tool steel cutting tools to be used in abrasive environments. In sputtering, a film of the target material is deposited on the substrate. During the sputtering process, a high voltage electric field is applied between the cathode target and an anode in a vacuum chamber containing a low pressure inert gas (such as argon). The potential causes gas ions to strike the cathode target dislodging target atoms and molecules, some of which impinge on, and adhere to, the substrate.
Fuel pellets coated with a boron containing burnable poison such as elemental boron, boron-10 isotope (the isotope of elemental boron having the burnable poison property), zirconium diboride, boron carbide, boron nitride, and the like suffer from varying degrees of moisture adsorption. For example, uranium dioxide fuel pellets coated with zirconium diboride, after manufacture, must be furnace dried in a time consuming operation and then loaded into the fuel rods in a low humidity glove box environment.